Exhaust gas aftertreatment devices, such as for example catalytic converters and particles filters, can comprise at least one monolith for the exhaust gas aftertreatment, which is retained in a housing with the help of a bearing mat enclosing the monolith in the circumferential direction. Such monoliths can be produced comparatively cost-effectively from ceramic materials. Ceramic materials are characterized by high temperature resistance. However, ceramic materials are comparatively brittle and have a thermal expansion coefficient which significantly differs from the thermal expansion coefficients of metals, which are usually employed for producing the housing. The bearing mat thus serves to protect the monolith from voltage peaks. On the other hand, the bearing mat has to be deformable elastically reversibly in order to be able to offset thermally-induced expansion effects. Furthermore, the bearing mat can also dampen vibrations and oscillations in order to thereby reduce the loading of the monolith. A further problem when using ceramic monoliths is production-related. While the metallic housing can be produced with comparatively close shape tolerances, the shape tolerances for producing ceramic monoliths are significantly greater. The bearing mats, too, can have geometries or densities which vary, in particular from lot to lot. In order to be able to fix the respective monolith with sufficient strength in the housing with the help of the respective bearing mat, a predetermined preload force however has to be realized with which the bearing mat can be pressed into a ring gap, which radially materializes between the monolith and a jacket of the housing. If compression is too weak, the monolith can axially move relative to the housing during operation. Furthermore, the aggressive hot exhaust gases during the operation of the exhaust gas aftertreatment device can attack and destabilize the bearing mat. If by contrast the compression is too severe, the bearing mat can no longer elastically absorb the thermal expansion effects, as a result of which the lifespan of the mounting or the retaining of the monolith in the housing is likewise greatly impaired. Additionally, the permissible stresses can then be exceeded on the monolith as a result of which the risk of damaging the monolith also arises. To sort this problem it is usual to individually adapt the housing with respect to its dimensions to the respective monolith in order to provide a desired gap width radially between the monolith and the jacket of the housing. The predetermined radial gap width then leads to a predetermined radial compression of the bearing mat in the assembled state, so that the bearing mat can optimally fulfill its functions.
Adapting the housing to the dimensions of the respective monolith can take place prior to inserting the monolith with the bearing mat in the jacket of the housing, so-called pre-sizing. Pre-sizing can be realized on the one hand in that a selection of housings with varying dimensions is kept in stock so that dependent on the respective monolith a housing that is suitable with respect to its dimensions can be installed. On the other hand it is possible to keep only one housing side in stock, which is matched to the largest monolith within the tolerance range. Since this standard housing is then too large, for most of the monoliths, a cross-sectional reduction has to be carried out before inserting the monolith in the jacket of the housing, which is possible with the help of suitable tools. During pre-sizing, introducing the monolith with the bearing mat in the housing is comparatively complex since during the axial insertion comparatively large shearing forces act on the bearing mat, as a result of which damages to the bearing mat can occur.
Alternatively to pre-sizing, a so-called post-sizing can also be carried out, during which the monolith is axially introduced with the bearing mat into a comparatively large-dimensioned jacket of a standard housing and with which subsequently, i.e. with the monolith inserted in the jacket and enveloped by the bearing mat, a cross-sectional reduction is carried out in order to individually adapt the dimensions of the jacket or of the housing to the dimensions of the respective monolith. The cross-sectional reduction for adapting the dimensions of the housing to the individual dimensions of the respective monolith can also be called “calibrating” of the housing, namely both during pre-sizing and also during post-sizing.
The housing usually consists of said material, which encloses the respective monolith with the bearing mat in the circumferential direction, and two funnels which are arranged at the face end of the jacket. Usually, the jacket and the two funnels are separate components so that the housing has to be assembled by attaching the two funnels to the jacket. Accordingly, at least one of the funnels has to be attached to the jacket after the inserting of the monolith. Usually, the respective funnel with a cylindrical connecting section can be plugged with the cylindrical jacket; either the funnel with its connecting section is plugged onto the jacket or into the jacket. Practically, jacket and funnel are fastened to one another by means of a closed circumferential weld seam in the circumferential direction.
Since the jacket through the calibrating can have varying cross sections, further problems for attaching the funnels arise since standard funnels are too large as a rule and weld seams are comparatively unstable for offsetting these dimensions. For reducing these problems it is possible in principle to also calibrate the funnels so that calibrated funnels can then be plugged with the calibrated jacket. Through this measure, a radial play between the respective face-end region of the jacket and the connecting section of the respective funnel overlapping said jacket can be reduced. Thus, the required weld seams can be configured smaller, which favors their strength.
In modern exhaust systems, in particular with smaller motor vehicles, there is frequently the desire to construct the respective exhaust gas aftertreatment device as compact as possible in the axial direction. This can be achieved in that within the housing the axial spacing between the monolith and the funnels is selected as small as possible. A further reduction of the dimensions can only be achieved in that the respective plugging region, in which the jacket and the respective funnel are plugged into one another, axially overlaps with an axial end region of the monolith. Because of this, new problems are created however since with plugged-in funnel the respective connecting section of the funnel then axially enters the ring gap between monolith and jacket, which impairs the installation space that is available for the bearing mat. With plugged-on funnel, however, an axial region of the jacket, in which the required calibrating can be carried out, is reduced. There is thus the risk altogether that the bearing mat can no longer optimally fulfill its retaining function.